Teacher sheet · Ages 15–16 · about 45–50 minutes
Deserts: water balance, energy and adaptation
Lesson page: echoesof.science/school/deserts/ages-15-16/ · Pupil worksheet: echoesof.science/school/deserts/ages-15-16/print/
Learning aims
- Compare definitions and explain why estimates of desert area differ.
- Explain the atmospheric causes of deserts (subtropical high pressure, rain shadow).
- Use unit conversions to compare rainfall and evaporation, and explain day–night temperature ranges with an energy budget.
- Measure rate of water loss and relate surface area to plant adaptations.
Lesson plan
| Time | Activity |
|---|---|
| 5 min | Starter: rank Antarctica, the Sahara and Brussels by yearly precipitation. Discuss what makes a desert. |
| 10 min | Read the text; unit conversions (inches to mm, mm to litres, °F ranges to °C). |
| 20 min | Surface-area practical with readings every 15 minutes. |
| 5 min | Avoid or endure? game. |
| 5 min | Quiz; discuss the different definitions. |
Materials for the experiment
- Three identical paper towels
- 10 ml measuring cylinder or syringe
- Electronic balance (0.1 g)
- Three Petri dishes or saucers
- Cling film and sticky tape
- Ruler
- Stopwatch
- Spreadsheet or graph paper
Safety: Use only water. Keep water away from the balance's plug and socket and wipe up spills. No heaters or lamps are needed.
Common misconceptions
| Pupils may think… | Actually… |
|---|---|
| A temperature range in °F converts like a temperature reading (subtract 32). | A difference converts by × 5/9 only: 50 °F range ≈ 28 °C range. |
| Deserts are caused by drought, so desertification is natural. | Desertification is usually driven by deforestation and human demands on semiarid land. |
| Shape itself matters, not area. | The rate per unit of exposed area is similar; less exposed area means slower loss. |
Worksheet answers
- 70 × 25.4 = 1,778 mm; 160 × 25.4 = 4,064 mm. 4,064 ÷ 250 ≈ 16 times the arid limit.
- Loss = 24.0 − 15.6 = 8.4 g in 1.5 h, so 5.6 g/h; per 100 cm² = 5.6 ÷ 3 ≈ 1.9 g/h.
- By day, clear skies let at least 90% of sunlight reach the ground, so it heats strongly. At night there are few clouds and little water vapour, so about 90% of the day's heat radiates away (against about 50% in moist places), and the temperature drops sharply.
- Give both figures with their definitions (for example arid = less than 250 mm, with or without polar deserts) and say that the difference comes from the definition, not a measurement error.
Quiz answers
- In Meigs' classification, how much rain does semiarid land get a year? 250–500 mm. Semiarid land is usually steppe, not desert.[1]
- Cairo gets 28 mm of rain a year. How many litres fall on each square metre? 28 litres. 1 mm on 1 m² equals 1 litre.[2]
- A daily temperature range of 50 °F is about how big in °C? About 28 °C. A temperature difference converts as × 5/9, without the 32 offset.[4]
- Why do the USGS and National Geographic give different shares of land as desert? They use different definitions of a desert. Counting polar and borderline areas or not changes the total.[1][3]
- Why does at least 90% of sunlight reach the ground in many deserts? High pressure brings very few clouds. Clear skies let most of the Sun's rays through.[4]
- How do ephemeral plants survive drought? They escape it by completing their life cycle in weeks after rain. Their seeds wait in the soil until the next rain.[2]
Going further
Collect a month of daily maximum and minimum temperatures for a desert city and for your own town from a national weather service, calculate the average daily range for each and explain the difference.
Sources used in this lesson
- [1] Deserts: Geology and Resources – What is a desert? (U.S. Geological Survey (General Interest Publication)): reliability High
- [2] Mission: Biomes – Desert (NASA (Mission: Biomes)): reliability High
- [3] Deserts explained (National Geographic Society (Education resource library)): reliability Medium-high
- [4] Deserts (Joshua Tree National Park) (U.S. National Park Service): reliability High
- [5] Animal fact sheet: Merriam's kangaroo rat (Arizona-Sonora Desert Museum): reliability High
- [6] Desert: Biota (Encyclopaedia Britannica): reliability High
Full list with links and a note on each source's reliability: echoesof.science/school/deserts/sources/
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